The present invention relates to a start-up range neutron monitor (SRNM) system in a nuclear power plant particularly for suppressing contamination of external noise.
A nuclear power plant includes a reactor building in which is installed a reactor containment vessle in which a reactor is disposed.
A structure of a known start-up range neutron monitor system in a nuclear power plant is shown in FIG. 8, and the known SRNM system of FIG. 8 comprises a neutron detector 1 arranged in a reactor, coaxial cables 3 and 4 for transferring signals detected by the neutron detector 1 to a signal processing unit 2a disposed inside a monitor 2 arranged in a central control chamber, and a preamplifier 5 disposed between these cables 3 and 4. Namely, the neutron detector 1 is operably connected to the preamplifier 5 in the reactor building through the coaxial cable 3 connecting the neutron detector 1 and the preamplifier 5 by penetrating inside the reactor containment vessel and the coaxial cable 4 connects the preamplifier 5 and the signal processing unit 2a. These coaxial cables 3 and 4 are composed of cores 3a and 4a and outer sheaths 3b and 4b for earthing, respectively. An earth circuit has one point earth structure earthed through the signal processing unit 2a.
In the known start-up range neutron monitor system of FIG. 8, electric pulse signals in response to thermal neutrons in the start-up range in the reactor are detected. The thus detected signal has a weak magnitude, so that the detected amplified by the preamplifier 5 and then treated with by the signal processing unit 2a of the monitor 2.
However, since the known SRNM system has a structure in which, as described above, the neutron detector 1 and the preamplifier 5 are connected through the coaxial cable 3, when the external noise is transferred to the coaxial cable 3, an S/N (signal/noise) ratio of the weak signal is extremely lowered by the external noise, thus being inconvenient.
This problem will be explained in detail with reference to FIGS. 9 and 10.
Supposing that the external noise is invaded into the coaxial cable 3 on the input side of the preamplifier 5 and a noise current I.sub.N is caused by the external noise in the outer sheath 3b, a circuit in such case will be modeled as that shown in FIG. 9 and an equivalent circuit is shown in FIG. 10, in which reference numeral 1' denotes a detection signal source by means of the neutron detector 1.
Referring to FIGS. 9 and 10, the start-up range neutron monitor system has, as a whole, one point earth structure in the central control chamber, and the neutron detector 1 has an isolated, i.e. non-earthed, structure. For this reason, when an impedance of the coaxial cable 3 and the circuit is supposed to R.sub.C, a noise voltage V.sub.12, caused between both poles P.sub.1 and P.sub.2 of the preamplifier 5 is represented as EQU V.sub.12 =I.sub.N .multidot.R.sub.C ( 1)
That is, even if the noise current I.sub.N be weak, the noise voltage V.sub.12 becomes R.sub.C times of the current I.sub.N, so that the S/N ratio of the detected weak signal of the neutron detector 1 is lowered, thus requiring a complicated signal processing circuit of the monitor 2 disposed on the output side of the preamplifier 5 and an increased load for calculation of the signal processing, thus imparting adverse influence on the signal treatment.